G. Mégie

The CALIPSO Mission: A Global 3D View of Aerosols and Clouds

Aerosols and clouds have important effects on Earths climate through their effects on the radiation budget and the cycling of water between the atmosphere and Earths surface. Limitations in our understanding of the global distribution and properties of aerosols and clouds are partly responsible for the current uncertainties in modeling the global climate system and predicting climate change. The CALIPSO satellite was developed as a joint project between NASA and the French space agency CNES to provide needed capabilities to observe aerosols and clouds from space. CALIPSO carries CALIOP, a two-wavelength, polarization-sensitive lidar, along with two passive sensors operating in the visible and thermal infrared spectral regions. CALIOP is the first lidar to provide long-term atmospheric measurements from Earths orbit. Its profiling and polarization capabilities offer unique measurement capabilities. Launched together with the CloudSat satellite in April 2006 and now flying in formation with the A-train satellite constellation, CALIPSO is now providing information on the distribution and properties of aerosols and clouds, which is fundamental to advancing our understanding and prediction of climate. This paper provides an overview of the CALIPSO mission and instruments, the data produced, and early results.

An overview of the SOLVE/THESEO 2000 campaign

[1]xa0Between November 1999 and April 2000, two major field experiments, the Stratospheric Aerosol and Gas Experiment (SAGE) III Ozone Loss and Validation Experiment (SOLVE) and the Third European Stratospheric Experiment on Ozone (THESEO 2000), collaborated to form the largest field campaign yet mounted to study Arctic ozone loss. This international campaign involved more than 500 scientists from over 20 countries. These scientists made measurements across the high and middle latitudes of the Northern Hemisphere. The main scientific aims of SOLVE/THESEO 2000 were to study (1) the processes leading to ozone loss in the Arctic vortex and (2) the effect on ozone amounts over northern midlatitudes. The campaign included satellites, research balloons, six aircraft, ground stations, and scores of ozonesondes. Campaign activities were principally conducted in three intensive measurement phases centered on early December 1999, late January 2000, and early March 2000. Observations made during the campaign showed that temperatures were below normal in the polar lower stratosphere over the course of the 1999–2000 winter. Because of these low temperatures, extensive polar stratospheric clouds (PSC) formed across the Arctic. Large particles containing nitric acid trihydrate were observed for the first time, showing that denitrification can occur without the formation of ice particles. Heterogeneous chemical reactions on the surfaces of the PSC particles produced high levels of reactive chlorine within the polar vortex by early January. This reactive chlorine catalytically destroyed about 60% of the ozone in a layer near 20 km between late January and mid-March 2000, with good agreement being found between a number of empirical and modeling studies. The measurements made during SOLVE/THESEO 2000 have improved our understanding of key photochemical parameters and the evolution of ozone-destroying forms of chlorine.

Simultaneous nighttime Lidar measurements of atmospheric sodium and potassium

Abstract Simultaneous measurements of the nighttime atmospheric sodium and potassium layer have been performed over a period of one year, using a lidar facility set up at the Haute Provence Observatory. A detailed description of the calibration method is given together with the estimated accuracy of the experiment. The similarity observed in the behaviour of the spatial parameters of both layers indicates that the same processes are responsible for their day-to-day variations. However, the long term behaviour of the sodium and potassium total column abundances is very different: whereas the sodium one shows the usual seasonal variation already observed, no marked increase is seen in the potassium abundance during the year. The abundance ratio of these two elements varies then between a low summer value (∼10) and a high winter value (∼50). Two different origins can then be assumed for the alkalis in the upper atmosphere: a meteoritic one constant over the year and a terrestrial source due to the vertical transport of particles at high latitudes which only works in winter. Confirmation of this hypothesis can be found in the lack of sodium seasonal variation as observed at low latitudes.

Monitoring of ozone trend by stellar occultations: the GOMOS instrument

Abstract As a part of the payload of the first European Polar Platform, the GOMOS instrument has been proposed by a group of 25 scientists from six countries. It consists of a telescope feeding two spectrographs, mounted on a dedicated steerable platform. The transmittance of the atmosphere between 250 and 675 nm is measured by comparing the spectrum of a star outside the atmosphere, and through it. The ozone tangential column is determined from its UV and Chappuis band absorption. This self-calibrated method is particularly well suited for the study of ozone long term trend. The altitude of each single measurement is precisely known (± 50 m), independently of altitude uncertainties. About 25 stellar occultations per orbit, and 350 per day, spread over all latitudes can be performed from 90 km down to 15–20 km of altitude. NO 2 , NO 3 , H 2 O, T(z) and aerosols are also simultaneously determined, important parameters associated to the ozone equilibrium. The ability to measure ozone long-term trends is calculated.

Operational trace gas retrieval algorithm for the Infrared Atmospheric Sounding Interferometer

[1]xa0The Infrared Atmospheric Sounding Interferometer (IASI) is a nadir-viewing remote sensor due for launch on board the European Metop satellites (to be launched in 2005, 2010, and 2015). It is dedicated to the study of the troposphere and the lower stratosphere to support operational meteorology as well as atmospheric chemistry and climate studies. For this purpose, it will record high resolution atmospheric spectra in the thermal infrared, allowing the measurement of several infrared absorbing species. This paper describes the clear-sky retrieval scheme developed in the framework of the preparation of the IASI mission for the operational, near real time, retrieval of O3, CH4, and CO concentrations. It includes the inversion module, based on a neural network approach, as well as an error analysis module. The studies undertaken on test simulations have shown that a performance of the order of 1.5%, 2%, and 5% for the retrieval of total columns of O3, CH4, and CO, respectively, can be achieved, and of the order of 28%, 15%, and 9% for the retrieval of partial columns of O3 between the surface and 6, 12, and 16 km high, respectively. The efficiency of the algorithm is demonstrated on the atmospheric measurements provided by the Interferometric Monitor for Greenhouse Gases (IMG)/ADEOS, allowing to obtain the first remote-sensing simultaneous distributions of ozone and its two precursors, CO and CH4.

Comparative lidar study of the optical, geometrical, and dynamical properties of stratospheric post‐volcanic aerosols, following the eruptions of El Chichon and Mount Pinatubo

The spatiotemporal evolution of aerosols formed from precursors injected into the stratosphere by major volcanic eruptions, such as those of El Chichon in 1982 and Mount Pinatubo in 1991, has been studied using a ground-based lidar system located at the Observatoire de Haute-Provence (OHP) in southern France (44°N, 5°E). From the inversion of the lidar signals the optical, geometrical and dynamical properties of the particles have been determined as a function of time after the eruption. In immediate post-volcanic conditions, when the optical thickness of particles in the stratosphere is largely enhanced, an estimate of the aerosols backscatter phase function has been evaluated directly from the lidar measurements, using a size-distribution model adjusted to in situ balloon measurements. The precision of this determination lies in the ±15% range. Values of the mean radius of the particles, of their integrated content, surface areas, and sedimentation velocities are then derived from the systematic lidar measurements performed at OHP. These values are compared for the two major volcanic eruptions which have occurred over the last decade. Although the injection of sulphur dioxide was twice as large for the Mount Pinatubo eruption as compared to the El Chichon case, the diffusion of the cloud in the two hemispheres due to the interaction of the particular phase of the quasi-biennal oscillation with several other dynamical processes at the time of the eruption, led to the observation of similar values for the aerosol content over the Observatoire de Haute-Provence in the months just following the two events. However, the residence time of the particles in atmospheric layers below 20 km are 4 months longer after the Mount Pinatubo eruption, caused by the observed difference in the initial vertical distribution of the aerosol cloud.

Multiwavelength lidar for ozone measurements in the troposphere and the lower stratosphere.

To study the ozone spatial and temporal evolution in the atmosphere, lidar systems have proved to be adequate but have remained complex. We define in this paper the main characteristics of a UVDIAL system for ground based and airborne ozone measurements in the troposphere and the lower stratosphere both for daytime and nighttime operation. A multiwavelength lidar system using either Rayleigh/Mie signals or the Raman nitrogen signal, is discussed as a way to efficiently correct the ozone measurements from the systematic bias due to aerosol and other interference gases (i.e. SO(2)) in the lower troposphere. Two types of lasers (solid state and excimer) are compared, as both lasers are suitable for long term field operation and airborne use.

Polar vortex evolution during the 2002 Antarctic major warming as observed by the Odin satellite

In September 2002 the Antarctic polar vortex split in two under the influence of a sudden warming. During this event, the Odin satellite was able to measure both ozone (O3) and chlorine monoxide (ClO), a key constituent responsible for the so-called “ozone hole”, together with nitrous oxide (N2O), a dynamical tracer, and nitric acid (HNO3) and nitrogen dioxide (NO2), tracers of denitrification. The submillimeter radiometer (SMR) microwave instrument and the Optical Spectrograph and Infrared Imager System (OSIRIS) UV-visible light spectrometer (VIS) and IR instrument on board Odin have sounded the polar vortex during three different periods: before (19–20 September), during (24–25 September), and after (1–2 and 4–5 October) the vortex split. Odin observations coupled with the Reactive Processes Ruling the Ozone Budget in the Stratosphere (REPROBUS) chemical transport model at and above 500 K isentropic surfaces (heights above 18 km) reveal that on 19–20 September the Antarctic vortex was dynamically stable and chemically nominal: denitrified, with a nearly complete chlorine activation, and a 70% O3 loss at 500 K. On 25–26 September the unusual morphology of the vortex is monitored by the N2O observations. The measured ClO decay is consistent with other observations performed in 2002 and in the past. The vortex split episode is followed by a nearly complete deactivation of the ClO radicals on 1–2 October, leading to the end of the chemical O3 loss, while HNO3 and NO2 fields start increasing. This acceleration of the chlorine deactivation results from the warming of the Antarctic vortex in 2002, putting an early end to the polar stratospheric cloud season. The model simulation suggests that the vortex elongation toward regions of strong solar irradiance also favored the rapid reformation of ClONO2. The observed dynamical and chemical evolution of the 2002 polar vortex is qualitatively well reproduced by REPROBUS. Quantitative differences are mainly attributable to the too weak amounts of HNO3 in the model, which do not produce enough NO2 in presence of sunlight to deactivate chlorine as fast as observed by Odin.

Lidar measurements of ozone vertical profiles

Remote measurements of trace constituents using an active technique such as lidar have been made possible for the rapid development of powerful tunable laser sources. This paper, originally presented at the OSA Topical Meeting on Optical Remote Sensing of the Atmosphere, in January 1985, illustrates the differential absorption lidar technique used for the measurement of the ozone vertical distribution in the troposphere and the atmosphere.

Atmospheric CH 4 and H 2 O monitoring with near-infrared InGaAs laser diodes by the SDLA, a balloonborne spectrometer for tropospheric and stratospheric in situ measurements

The Spectromètre à Diodes Laser Accordables (SDLA), a balloonborne spectrometer devoted to the in situ measurement of CH(4) and H(2)O in the atmosphere that uses commercial distributed-feedback InGaAs laser diodes in combination with differential absorption spectroscopy, is described. Absorption spectra of CH(4) (in the 1.653-microm region) and H(2)O (in the 1.393-microm region) are simultaneously sampled at 1-s intervals by coupling with optical fibers of two near-infrared laser diodes to a Herriott multipass cell open to the atmosphere. Spectra of methane and water vapor in an altitude range of approximately 1 to approximately 31 km recorded during the recent balloon flights of the SDLA are presented. Mixing ratios with a precision error ranging from 5% to 10% are retrieved from the atmospheric spectra by a nonlinear least-squares fit to the spectral line shape in conjunction with in situ simultaneous pressure and temperature measurements.